You can download bicycle-oriented OSM maps for Garmin from
http://garmin.openstreetmap.nl/. Select the "routable bicycle" option, pick the country or tiles you want, and it will either immediately show you the download link for the zipped gmapsupp file if you have selected a predetermined country, or it will create the gmapsupp file download for you if you have chosen custom tiles, and email you the download link when it has built the gmapsupp.img file. So if your device only allows a single *.img file, You can create one covering whatever map areas you desire.
You can also merge multiple maps into a single gmapsupp.img file using mkgmap. See the Openstreetmap wiki:
http://wiki.openstreetmap.org/wiki/OSM_Map_On_GarminMost modern devices are not restricted to a single *.img file - they will open any img file placed in the Garmin directory. Again, see the Openstreetmap wiki for details.